1. Technical Field
The present invention relates to a technology for driving an electro-optical element such as a liquid crystal element.
2. Related Art
In a liquid crystal device, a driving method in which a vertical electric field is applied in a light emission direction to liquid crystal and a driving method in which a horizontal electric field is applied in a direction perpendicular to the light emission direction to liquid crystal are known. When a DC voltage is applied to liquid crystal, image quality deterioration such as burn-in may be caused. As a driving method, an AC driving method in which an AC voltage is applied to liquid crystal is used.
In a driving method in which a vertical electric field is applied to liquid crystal, for example, a pixel circuit shown in FIG. 22 is used. In this pixel circuit, when a scan signal Y becomes of a high level, a transistor Tr is turned on, and a data voltage Vdata supplied via a data line 10 is applied to a liquid crystal element 5 and is held in a hold capacitor C. The liquid crystal element 5 is configured by inserting liquid crystal LC between a pixel electrode 5a and a common electrode 5b. The transistor Tr and the pixel electrode 5a are formed on a device substrate, and the common electrode 5b is formed on a counter substrate. The device substrate and the counter substrate are adhered to each other with a gap therebetween and liquid crystal is injected therebetween. The common electrode 5b formed on the counter substrate also functions as a plurality of pixel circuits and is applied with a common voltage VCOM. In such a circuit configuration, a period when the data voltage Vdata is higher than the common voltage VCOM and a period when the data voltage Vdata is lower than the common voltage VCOM are alternately repeated so as to apply an AC voltage to the liquid crystal LC.
In a pixel circuit using the horizontal electric field driving method, a first electrode and a second electrode are formed on the device substrate on which switching transistors are formed. A technology for respectively applying signals, which are image signals having a positive polarity and a negative polarity with respect to a fixed voltage and have an identical absolute value, to the first electrode and the second electrode is known (for example, JP-A-2003-149654 (paragraph number 0033)).
However, in the pixel circuit shown in FIG. 22, a timing when the scan signal Y is switched from a high level to a low level, a push-down phenomenon in which the voltage applied to the liquid crystal LC is reduced occurs. The push-down phenomenon occurs because charges written into the liquid crystal element 5 escape from a scan line 20 via a coupling capacitance between the scan line 20 and the liquid crystal LC in a moment when the scan signal Y applied to the scan line 20 is changed from a voltage level for turning on the transistor Tr to a voltage level for turning off the transistor Tr. The coupling capacitance mainly includes a capacitance component Cgd between a gate electrode of the transistor Tr and the liquid crystal LC and a capacitance component Cgd′ between the scan line 20 and the pixel electrode 5a. Between them, the capacitance component Cgd varies in accordance with a voltage Vgd applied between the gate electrode and a drain electrode and the capacitance component Cgd increases as the voltage Vgd applied between the gate electrode and the drain electrode increases. As the coupling capacitance increases, the voltage applied to the liquid crystal LC decreases.
A drop in voltage due to the push-down phenomenon will be described in detail with reference to FIG. 23. For example, in the pixel circuit shown in FIG. 22, the data voltage Vdata having the positive polarity with respect to the common voltage VCOM is applied to the first electrode in a first frame period F1, and the data voltage Vdata having the negative polarity with respect to the common voltage VCOM is applied to the first electrode in a second frame period F2. In this case, the voltage applied to the liquid crystal LC decreases in accordance with the voltage Vgd between the gate and drain electrodes of the transistor Tr. In this example, the liquid crystal LC is arranged in a normally white mode and ΔV1<ΔV2<ΔV3<ΔV4 is realized. For example, when a black level is displayed, the voltage applied to the liquid crystal LC decreases by ΔV1 in the first frame period F1 and the voltage applied to the liquid crystal LC increases by ΔV4 in the second frame period F2. That is, the voltage applied to the liquid crystal LC is shifted to a white side in the first frame period F1 and the voltage applied to the liquid crystal LC is shifted to a black side in the second frame period F2.
Accordingly, when the pixel circuit shown in FIG. 22 is employed, in order to correct the shift of the voltage applied, to the liquid crystal LC, a DC voltage needs to be prevented from being applied to the liquid crystal LC by controlling the common voltage VCOM. Gamma correction process needs to be switched according to the polarity of the applied voltage. Even in the pixel circuit using the horizontal electric field driving method, since the signals which are image signals having a positive polarity and a negative polarity with respect to a fixed voltage and have an identical absolute value are respectively applied to the first electrode and the second electrode, the same problem is caused.